1. Field of the Invention
The present invention relates to a dispersion monitoring method and apparatus as well as a dispersion slope temperature dependency compensation method and apparatus, more specifically, relates to a method of monitoring a dispersion variation amount on a transmission optical fiber in a wavelength division multiplexing (WDM) optical transmission system and a compensation method of a dispersion of a transmission line.
2. Description of the Related Art
At present, research/development has been promoted on optical communication of high speed/large capacity for carrying a photonic network in the future. In realizing such an optical communication, there is compensation of a dispersion of a transmission line as one of important problems. As shown in FIG. 1, in an optical transmission system, an optical signal (A in the figure) generated from a transmitter 1 is transmitted to a transmission optical fiber 2 and is transmitted to a receiver 5 while being optically amplified by optical repeaters 3 at certain distances. The optical signal immediately after transmission is deteriorated in a waveform thereof by dispersions of the transmission optical fiber 2 and the optical repeaters 3 (B in the figure). When the optical signal is directly received, an error is produced in reading the signal by interference between contiguous optical pulses. Further, the more increased the velocity of the optical signal, the more reduced the 1 time slot (time width occupied by 1 bit) and therefore, the more increased is the influence on a transmission characteristic by a dispersion D (ps/nm). Therefore, there is a demand for a dispersion compensation means 4 for compensating the dispersion of the transmission line (transmission optical fiber and optical repeater).
What poses a problem here is that a dispersion of the transmission optical fiber is varied by temperature. In the case of a 1.3 μm zero dispersion fiber (single mode fiber: SMF) or a dispersion-shifted fiber, it is known that the dispersion is changed by the following equation by shifting a zero dispersion wavelength by a temperature change ΔT(deg) (K. S. Kim and M. E. Lines, “Temperature dependence of chromatic dispersion in dispersion-shifted fibers: Experiment and analysis”, J. Appl. Phys., Vol. 73, No. 5, pp. 2069-2074, 1993).ΔD=Z·S·L·ΔT  (1)where ΔD(ps/nm) designates a dispersion change amount, Z(nm/deg) designates a temperature constant of zero dispersion wavelength shift, S(ps/nm2/km) designates a dispersion slope (wavelength differential of dispersion) and L(km) designates an optical fiber length.
Heretofore, a variation amount of the dispersion by the temperature change has been regarded as constant without depending on the wavelength. Therefore, according to an adaptive dispersion equalization technology which has been proposed until present time, the variation of the dispersion is equalized among respective channels by the same amount (T. Inui et al. “Adaptive dispersion slope equalizer using a nonlinearly chirped fiber Bragg grating pair with a novel dispersion detection technique”, IEEE Photon. Technol. Lett., vol. 14, no. 4, P. 549 (2002), H. Ooi et al., “40-Gbit/s WDM automatic dispersion compensation with virtually imaged phased array (VIPA) variable dispersion compensators”, IEICE Trans. Commun., vol. E85-B, no. 2, p. 463 (2002)).
A reverse dispersion fiber (RDF) which has been developed recently (K. Mukasa et. al., “Novel network fiber to manage dispersion at 1.55 μm with combination of 1.3 μm zero dispersion single mode fiber”, ECOC'97, 1, p. 127 (1997)) is a fiber in which a dispersion/a dispersion slope are provided with signs reverse to the sign of the 1.3 μm zero dispersion optical fiber (single mode fiber: SMF) and absolute values thereof are near to those of SMF. By combining RDF with SMF, a transmission line in which both of dispersion and dispersion slope are near to zero can be realized. Therefore, a transmission line combined with SMF and RDF is used in WDM transmission (K. Yonenaga et al., “Dispersion-compensation-free 40 Gbit/s×4-channel WDM transmission experiment using zero-dispersion flattened transmission line”, OFC'98, PD20 (1998), E. Yamada et al., “106 channel×10 Gbit/s, 640 km DWDM transmission with 25 GHz spacing with supercontinuum multi-carrier source”, Electron. Lett., vol. 37, p. 1534 (2001)), and in ultra high speed OTDM transmission (M. Nakazawa et al., “1.28 Tbit/s−70 km OTDM transmission using third- and fourth-order simultaneous dispersion compensation with a phase modulator”, Electron Lett., vol. 36, p. 2027 (2000)). Further, RDF is utilized not simply as a transmission medium but a fiber for Raman amplification by utilizing the fact that a core diameter thereof is smaller than that of SMF or a dispersion-shifted fiber (H. Kawakami et al., “Highly efficient distributed Raman amplification system in a zero-dispersion flattened transmission line”, OA&A'99, ThB5 (1999), E. Yamada et al., “106 channel×10 Gbit/s, 640 km DWDM transmission with 25 GHz spacing with supercontinuum multi-carrier source”, Electron Lett., vol. 37, p. 1534 (2001)).
Although it has been known that the dispersion of the optical fiber is provided with temperature dependency, the temperature dependency of the dispersion slope has not been investigated. However, it has been known that a fiber having negative dispersion/dispersion slope as in RDF is provided with temperature dependency of the dispersion slope, that is, the wavelength dependency of the temperature dependency of the dispersion which cannot be disregarded. When a high speed/wide bandwidth WDM transmission line is constructed by SMF and RDF without taking the characteristic into consideration, a dispersion having a different value among channels is produced by temperature change exceeding an allowable dispersion value depending on channels.
<Temperature Dependency of Dispersion Slope of RDF>
FIG. 2A and FIG. 2B schematically show respective dispersion curves (dispersion vs wavelength) of SMF and RDF. In the case of the fiber having a negative dispersion slope of a dispersion as in RDF, structural dispersion is adjusted by devising a refractive index profile and dispersion on a side of a long wavelength is increased in a negative direction. Therefore, when dispersion vs wavelength of the fiber is plotted, whereas dispersion vs wavelength of SMF substantially becomes a linear line, that of RDF becomes a line having a radius of curvature. This can be expressed also as an absolute value of a quartic dispersion (wavelength differential of dispersion slope) is larger than that of SMF.
In this case, when changes of dispersions at wavelengths λ1 and λ2 in the case of changing temperature of the fiber from T1 to T2 are observed, when the dispersion curves are shifted by temperature change, in the case of SMF of FIG. 2A, magnitudes of change amounts ΔD1SMF and ΔD2SMF at λ1 and λ2 are almost the same. In contrast thereto, in the case of a fiber having a dispersion characteristic having a radius of curvature as in RDF, as shown in FIG. 2B, magnitudes of change amounts ΔD1RDF and ΔD2RDF at λ1 and λ2 are clearly different from each other. The fact represents that the wavelength dependency which cannot be disregarded is present in the temperature dependency of the dispersion of RDF (the fact may be regarded as the dispersion slope of RDF is provided with temperature dependency).
FIG. 3 shows curves produced by measuring temperature dependency constants of dispersions with regard to SMF and RDF and the temperature dependency constants are plotted with respect to four wavelengths. An inclination of a linear line produced by connecting the four points becomes equivalent to temperature dependency constants of the dispersion slopes. Whereas the value is 1.79×10−6 ps/km/nm2/deg for SMF, the value for RDF is 1.48×10−5 ps/km/nm2/deg which is larger than the value of SMF by one order of magnitude.
<Influence Effected by Temperature Dependency of Dispersion Slope>
When a wavelength bandwidth of an optical signal in an optical transmission system is defined as Δλ(nm) and a temperature dependency constant and a length of a dispersion slope of a transmission optical fiber are defined respectively as αT(ps/nm2/km/deg) and L(km) and a temperature variation is defined as ΔT(deg), a dispersion change amount difference ΔD(ps/nm) is represented by the following equation.ΔD=αT·L·ΔT·Δλ  (2)
Therefore, for example, when a transmission line is provided with a value αT=1.48×10−5 (ps/nm2/km/deg) similar to that of RDF and the optical fiber length is set to L=1000 (km), the temperature change is set to ΔT=50 (deg) and the wavelength bandwidth is set to Δλ=100 (nm), the dispersion change amount difference becomes ΔD=74.0 (ps/nm). That is, even when the dispersion is set to 0 in a total range of the wavelength bandwidth of 100 nm at an initial time of operating a WDM transmission system and a variation amount of the dispersion is compensated by an adaptive type dispersion equalizer by the same amount over a total wavelength bandwidth, by the temperature dependency of the dispersion slope, a difference of a dispersion of 62.5 (ps/nm) is produced between channels of the shortest wavelength and the longest wavelength. In this case, application to a WDM transmission system (allowable dispersion of about 40 ps/nm) of 40 Gbit/s/ch becomes difficult.
As described above, a dispersion of a value which differs among channels is produced by a temperature change in WDM transmission of high speed/wide bandwidth by the temperature dependency of the dispersion slope provided to the optical fiber. Therefore, even when the variation amount of the dispersion is compensated by the adaptive type dispersion equalizer by the same amount over a total wavelength bandwidth, there poses a problem that the variation amount exceeds the allowable dispersion value depending on channels.